In the application of electric motors to operate various types of loads, it is not uncommon to match a load to equal very closely the starting torque of the motor. If the motor is of a permanent magnet bi-directional type, it has been found that the motor sometimes will start in the wrong direction in the event that its directionally reliable starting torque is inadvertently exceeded.
Application of permanent magnet bi-directional electric motors to drive loads, such as ventilating dampers, is quite common. It is desirable to use as small a motor as possible to drive the damper. As such, a particular damper will normally be matched with a motor that has just a sufficient operating torque to correctly operate the damper. Sometimes a damper will become wedged or loaded so that the motor can not easily operate the damper in the desired direction. When this occurs the damper motor reverses its operation, thereby operating in the wrong direction. This type of operation can be very detrimental, and normally motors have been oversized in order to avoid this problem. Oversizing of motors is a rather expensive expedient.